The present disclosure relates to semiconductor devices used in the microwave band and the millimeter-wave band, and more particularly to high-output power amplifiers.
With the recent higher-speed, larger-amount data communication, it is increasingly requested to increase the operating frequency and output of power amplifiers used in applications in the communication field, etc. In this relation, vigorous efforts are being made toward enhancement in the performance of transistors that are key devices of the power amplifiers.
A power amplifier used in the microwave band and the millimeter-wave band has a transistor such as a field effect transistor (FET) and matching circuits for matching a signal with the input/output impedances of the transistor. A general transistor has a number of linear fingers arranged in a comb shape. To achieve higher output, it is generally attempted to increase the total gate width by increasing the number of linear fingers of the FET and by combining some fingers into a unit to implement a multi-cell transistor.
As the size of the transistor is made larger, it becomes no more negligible in comparison with the millimeter-wave wavelengths. Therefore, the transistor must be handled as a distributed constant element. Specifically, when the size of the transistor is about one twentieth of the wavelength or larger, the transistor must be handled as a distributed constant element.
When the element size becomes larger than the above criterion for achievement of higher output, it is necessary to provide circuits for phase matching of a high-frequency signal at input/output portions of the transistor. Specifically, a circuit having a function of dividing the high-frequency signal for supply of power to the transistor and a circuit having a function of combining output signals of the transistor are provided at the input/output portions.
FIG. 12 shows an example (e.g., see Yukio Ikeda, et al, C-Band High-Output, High-Efficiency GaAs FET Amplifier, Shingaku-giho (Technical Report of IEICE), MW-88-52, 1-5 (1988), Japanese Patent Publication No. H07-307626, Japanese Patent Publication No. 2008-022235, and Japanese Patent Publication No. 2001-185966). In FIG. 12, a two-stage impedance transformer made of microstrip lines is used as the input/output matching circuits. A microwave power amplifier of FIG. 12 includes a FET 1 and an input matching circuit 5 and an output matching circuit 8 respectively placed at input and output portions of the FET 1. Metal wires 9 electrically connect the input matching circuit 5 with the FET 1 and connect the output matching circuit 8 with the FET 1. The input matching circuit 5 is made of a microstrip line 2 formed on a dielectric substrate 3 made of alumina, etc., and an input terminal 4 is connected to one end of the microstrip line 2. The length and characteristic impedance of the microstrip line 2 are set at values with which the input impedance of the FET 1 can match with the impedance of the power supply connected to the input terminal 4. Normally, the length is set to be a quarter wavelength of a desired frequency, and the characteristic impedance is set to be the geometric mean value of the input impedance of the FET 1 and the power supply impedance. In this way, the input matching circuit 5 is a one-stage impedance transformer made of the microstrip line 2 that serves to match the input impedance of the FET 1 with the power supply impedance and has a length of a quarter wavelength.
The output matching circuit 8 is made of a microstrip line 6 formed on the dielectric substrate 3, and an output terminal 7 is connected to one end of the microstrip line 6. The output matching circuit 8 is provided to match the output impedance of the FET 1 with the load impedance connected to the output terminal 7. The length of the microstrip line 6 is set to be a quarter wavelength, and the characteristic impedance is set to be the geometric mean value of the output impedance of the FET 1 and the load impedance. In this way, like the input matching circuit 5, the output matching circuit 8 is a one-stage impedance transformer made of the microstrip line 6 having a length of a quarter wavelength.
A microwave signal input at the input terminal 4 of the microwave power amplifier passes through the input matching circuit 5 to be supplied to the FET 1. The supplied microwave signal is amplified by the FET 1 and output at the output terminal 7 through the output matching circuit 8. Thus, in the microwave power amplifier, the FET 1 having a gate width with which desired output power is obtained is used, and the input matching circuit 5 and the output matching circuit 8 that respectively match the input impedance of the FET 1 with the power supply impedance and the output impedance of the FET 1 with the load impedance are provided at the input/output portions of the FET1.
When the chip size of the FET 1 becomes too large to be negligible in comparison with the wavelength, a phase difference and an amplitude difference occur in a high-frequency signal that passes through the input matching circuit for the FET 1, then through various parts of the FET 1, and is output from the FET 1 as shown in FIGS. 13A-13C, reducing the combining efficiency. This prevents achievement of higher output and higher gain.
As one means for suppressing or reducing the phase difference and the amplitude difference in the high-frequency signal for the FET 1, the high-frequency signal is divided by dividing the microstrip line into a plurality of lines, to supply the signal with suppressed or reduced phase difference and amplitude difference to the FET. FIG. 14A shows an example of such an amplifier, which is configured as follows. A RF signal input at an input terminal 14 is divided through equivalent input matching circuits 2a, 2b, and 2c that have transmission lines formed on an alumina substrate and plate capacitors formed on a high-dielectric substrate to be supplied to FET chips 1. RF signals amplified by the FET chips 1 are combined through equivalent output matching circuits 6a and 6b that have transmission lines formed on the alumina substrate, to be output at an output terminal 15. Input-side resistors 10 (input-side inter-chip resistors) and output-side resistors 11 (output-side inter-chip resistors), which are thin film resistances formed on the dielectric substrate, are respectively provided in portions of the space between the transmission lines constituting the equivalent input matching circuits 2a, 2b, and 2c and in portions of the space between the transmission lines constituting the equivalent output matching circuits 6a and 6b, which are opposed in inter-chip closed loop circuits, and connected to the transmission lines.
As shown in FIG. 14B, by providing the input-side resistors 10 and the output-side resistors 11 in parallel in the loops, odd-mode oscillation power a between two chips and odd-mode oscillation power β between four chips can be absorbed by the input-side resistors 10 and the output-side resistors 11, achieving stabilization of the amplifier.